Heat-exchange jacket attachment to extrusion cylinder



HEAT-EXCHANGE JACKET ATTAomEN'r To ExTRUsIoN CYLINDER Filed April 6, 1961 Jan. 28, 1964 A. w. NELSON 2 Sheets-Sheet 1 Jan. 218, 1964 A. w. NELSON 3,119,149

HEAT-EXCHANG JACKET ATTACHMENT TO EXTRUSION CYLINDER Filed April 6, 1961 2 Sheets-Sheet 2 /0 ,34 J4 e@ ff/f 4f/ 40 25 j l IN1/Enron. /Y/Q/Y h/ /1/6/50/2 s BY '52 '52 42 @wey United States Patent O 3,119,149 HEAT-EXCHANGE JACKET ATTACHMENT T0 EXTRUSlN CYNDER Alden W. Nelson, W est Mystic, Conn., assignor, by mesne assignments, to @rompton il Knowles Corporation, Worcester, Mass., .a corporation of Massachusetts Filed Apr. o, wel, Ser. No. ltlll 2 Claims. (Cl. 18--12) This invention relates to extruders for plastics and the like, and more particularly to extruders with heated cylinders.

The present invention is concerned with extruders having solid cylinders of wear and stress resistant steel, and heat-exchange jackets of a type which are cast and clamped on the cylinders and provided with internal heating means and frequently also outer cooling fins. The jackets are customarily cast in complemental, usually semi-circular, sections from suitable metals of high heat conductivity, mostly aluminum, which are of low cost and may be cast, but unfortunately have coefficients of expansion which differ widely from that of the steel of the cylinders. Accordingly, while these prior jackets are highly desirable and advantageous due to their good heatexchange properties, low cost and easy installation and maintenance, their efficiency drops rather sharply and they even strain the operating parts of extruders at more elevated cylinder temperatures due to their different thermal expansion from that of the cylinders. Thus, despite the usual efforts to clamp these jackets to the cylinders as firmly as possible, the jackets gape locally from the cylinders, increasingly toward the bottoms thereof in the case of horizontally extending cylinders, when heating them to the higher temperatures required for certain plastics, with the result that the cylinders are, over peripheral portions thereof, sumciently hotter than over other peripheral portions thereof to respond to their different expansion thcreat by actual bowing. While this difference in temperature periphera'lly of the cylinders under these conditions is highly objectionable because it may well lead to a non-uniform consistency of the eX- trudate therein, it is even further objectionable because the ensuing bowing of the cylinders will permit the operational end pressures, usually many tons, on the feed screws therein fully to exert themselves in bending the latter to the ultimate, and thus subjecting their lands to rapid Wear, if not roughing, in the cylinders.

lt is the primary aim and object of the present invention to provide for the steel cylinders of extruders sectional heat-exchange jackets of cast and clamp-on type and of the same good heat-conductive metal or metals as the prior jackets of this type, which have all the aforementioned advantages, but none of the disadvantages, of these prior jackets.

it is, therefore, an object of the present invention to provide for the steel cylinders of extruders sectional heatexchange jackets of cast and clamp-on type which, despite their formation from aluminum or other metals of similarv heat conductivity and coefficients of expansion, will `at all, and even the highest, operational temperatures of the cylinders gape from the latter at the most so insignificantly that there occur neither temperature differentials in the cylinders which would lead to a non-uniform consistency of the extrudate therein, nor bowing of the cylinders which would lead to rapid Wear or roughing of the feed screws therein.

It is another object of the present invention to provide for the steel cylinders of extruders sectional heat-exchange jackets of the aforementioned cast and clamp-on type the complemental sections of which are at their confront ing parting surfaces slightly spaced from each other when mounted on the cylinders, and these jacket sections are 3,1 3,1429 Patented Jan. 28, 1954 rmly mounted on the cylinders by bolt-type braces of steel or other metals of similar strengthand low coefficients of expansion, which are applied to the jackets in such fashionl that they resist normal thermal expansion of the comple-mental sections thereof peripherally away from the cylinders and, instead, force them to bend and unbend on their thermal expansion and contraction to whatever extent is necessary to retain them at all operational and also non-operational temperatures in sufficiently uniform form-lit and, hence, engagement with their cylinders to preclude any significant temperature differentials peripherally of the latter.

it is a further object of the present invention to provide for the -steel cylinders of extruders sectional heatexchange jackets of this type, of which the `complemental sections are machined to form-lit the cold cylinders so as to be in optimum heat-exchange relation with the latter at the very start of the heat-up cycle of an operation, and the aforementioned bolt-type steel braces are of high tensile strength and are in the assembly of the jacket sections with the cold cylinders expanded and left expanded with-in their elastic limit by tightening them accordingly, so that they act as springs which exert on the jacket sections even at their full operational temperatures exceedingly large clamp-on yforces that contribute greatly to the aforementioned lretention of the jacket sections in substantially uniform form-tit and, hence, engagement with their cylinders at all temperatures of the latter.

Another object of the .present invention is to provide for the steel cylinders of extruders sectional heat-exchange jackets of this type of which the aforementioned bolt-type steel braces are arranged so that they not only resist most directly and with optimum :force thermal expansion of the jacket sections peripherally away from their cylinders over the very peripheral portions thereof which would thermally expand the most away from the cylinde-rs if unrestrained, but also exert optimum force on these same peripheral portions of the jacket sections to bend and unbend them on their thermal expansion and contraction for their substantially uniform form-fit and, hence, engagement with their cylinders at all temperatures of the latter. This is achieved by anchoring the bolt-type braces with their ends directly to the complemental jacket sections, and forming the braces with straight Shanks which connect the complemental jacket sections across their confronting parting surfaces and are of maximum lengths at which the larger masses of the complemental jacket sections, including the heating means therein, at and near their confronting parting surfaces are confined between the braces and cylinders.

Other objects and advantages will `appear to those skilled in the art from the following, considered in conjunction with the accompanying drawings.

In the accompanying drawings, in which certain modes of carrying out the present invention are shown for illustrative purposes:

FIG. 1 is a fragmentary longitudinal section through an extruder embodying the present invention;

FIGS. 2 and 3 are enlarged `cross-sections through the extruder as taken on die lines 2 2 and 3--3-, respectively, of FG. l;

FG. 4 is -a fragmentary side elevation o-f the extruder as viewed in :the direction of the arrow Il in FiG. 3;

FIG. 5 is a fragmentary top view of an extruder embodying the present invention in a modified manner; and

FIG. 6 is a cross-section through the modified extruder as taken on the line 6-6 of FlG. 5.

Referring to the drawings, and more particularly to FIGS. l to 4 thereof, the reference numenal lllfdesignates on extruder which comprises a cylinder .12 with a longitudinal axis x having a throat 14 at one end through which to introduce plastic material in a solid state as Well as a delivery head 16 at the other end from which to discharge the plasticized material through a die aperture (not shown), and ya power-driven feed screw 18 for forcing the plastic through the cylinder 12 and delivery head 156. The cylinder 12 is provided with external heatexchange jackets 20 which in the present instance extend substantial-ly throughout the length of the cylinder. rllhese jackets 20 are adapted to heat the cylinder so that the heat from the latter, and the heat generated in the plastic material by virtue of being worked by the driven screw 18, combine to plasticize the material. Each jacket 2i) comprises several, in this instance two, complemental sections 22 which are cast from a metal of good heat conductivity, with each section having a substantially semicircular wall 23 lwith internal heating means which in the present instance are cast-in Calrods 24 with external terminals (not shown) for connection with a current source. 'I'he jacket sections 22 have their inner surfaces 26 machined for preferred form-fit on the outer periphery 28 of the cylinder `12, and the mounted sections of each jacket surround the cylinder periphery except at the narrowly spaced confronting parting surfaces 39 of their walls 23. The Calrods 24 are confined in inner, preferably part-circular, zones z of the Walls 23 of the jacket sections 22 (FIGS. 2 and 3) and extend in usual serpentine fashion therein for substantially 4uniform heating of the walls of the jacket sections throughout. The jacket sections are in this instance also provided with external cooling ribs 31 which are cast integrally with the walls 23 and may be subjected to air -blasts yfor close regulations of the heat in the cylinder under various operating conditions of the extruder. The cooling ribs 31 on the walls 23 of the complemental jacket sections 22 are preferably of the same cross-sectional dimensions throughout and equally spaced from each other. For their mount on the cylinder 12 the complemental sections 22 of each jacket are releasably clamped on the cylinder periphery 28 by rod-type braces which in this instance are -bolts 32 with nuts 34.

The cylinder `12 is customarily of wear and stress resistant steel, and the jacket sections 22 are rusually of aluminum `by virtue of its good heat conductivity, relatively low cost and ready casting, though these jacket sections may be cast from any other metal or metals suitable for the purpose. In any event, the metal of the jacket sections 22, being other than steel which has grossly inadequate heat conductivity for the required heat exchange, has a coeicient of expansion which is greater than that of the steel of the cylinder, and which is twice that of the steel of the cylinder if the jacket sections are cast from aluminum as @the much preferred metal. Accordingly, the cylinder A12 and the jacket sect-ions 22 will have different thermal expansion in all directions. In order to permit differential thermal expansion lof the jacket sections 22 and cylinder 12 in the direction of the cylinder axis in natural fashion, the complemental sections of each jacket 20 are merely clamped on the cylinder periphery 28 as aforementioned, and successive jackets are at their adjacent ends 36 slightly spaced from each other (FIGS. 1 and 4). However, while differential expansion of the jackets 20 and of the cylinder 12 axially of the latter is thus rendered harmless, the remaining diierential expansion of these parts, principally peripherally thereof, tends to gape the complemental jacket sections 22 with their inner wall surfaces 26 away from the cylinder periphery 28 Iwith ensuing nonmniform heating of the cylinder peripherally thereof. Thus, while peripheral thermal expansion of the cylinder 12 will result in a slightly increased radius of its periphery 28, concurrent peripheral thermal expansion of the complemental jacket sections 22, unless restrained from pursuing its natural course, will result in a larger increase of the radius of their inner wall surfaces 26 than that of the radius of the cylinder jperiphery and ensuing separation of these inner wall surfaces from form-tit with the cylinder perirhers In accordance with the present invention, the rod-type braces 32, 34 are arranged to restrain the peripheral thermal lexpansion of the complemental jacket sections 22 from pursuing its natural course' and, instead, direct it into a different course on which the inner wall surfaces 26 remain in substantial form it with the cylinder periphery 28 at all operational and non-operational cylinder temperatures. To this end, the present exemplary straight bolts 32 are anchored with their ends to the respective jacket sect-ions 22 rand extend across the narrowly spaced confronting parting surfaces 30 thereof and linearly therebeyond in both directions for considerable distances to their anchored ends, with the bolts having high tensile strength and a lower coeicient of expansion than the metal of the jacket sections and being under permanent longitudinal tension of such large magnitude as to clamp these jacket sections to the cylinder periphery in substantially uniform form-fit therewith at all operational and non-operational cylinder temperatures. For secure anchorage of the bolts 32 to the complemental jacket sections 22, the latter are 'provided over peripheral end lengths of their walls 23 with outer integral lug formations 42 which are arranged in aligned pairs spaced axially of the cylinder (FIGS. 1 and 4) and receiving the bolts 32 with the lheads 38 of the latter and the nuts 34 resting, advantageously through intermediation of large washers 33, against the opposite surfaces 44 of the respective aligned lug formations 42. As to the lengths of the bolts 32 between their anchored ends, i.e., between their heads 38 and their nuts 34 (FIG. 3), they are preferably of maximum lengths at which they are at their nearest approach to the cylinder axis x spaced somewhat farther from the latter than the heater zones z rin these jacket sections on the lone hand, and they span near optimum peripheral lengths of the jacket sections (angles a in FIG. 3) at which the washers 33 me spaced outwardly, but not very appreciably, from the adjacent outer wall surfaces 40 of these jacket sections on the other hand. Hence, taking into consideration the usual proportionate cross-sectional dimensions of cylinders and the walls` of cast complemental heat-exchange jacket sections thereon within fairly well established limits, the bolts 32 and washers 33 will have the aforementioned preferential spacing from the cylinder axis x and from the outer wall surfaces 440 of the jacket sections 22, respectively, as long as the bolts extend from either of the confronting parting surfaces 30 of the latter which they cross to their nearest anchored ends for a distance at least in excess of half the radius of the cylinder periphery 28.

In thus arranging the bolts 32 and tightening their nuts 34, they resist most directly and with optimum force normal peripheral thermal expansion of the walls 23 of the complemental jacket sections 22 `over optimum peripheral lengths thereof which, if unrestrained, would respond to thermal expansion by separating farthest from the cylinder periphery, namely, the peripheral Wall lengths spanned 4by the bolts 32. In accordance with the present invention, the bolts 32 and nuts 34, which are preferably of steel and have a coeiiicient of expansion smaller than that of the metal of the cast jacket sections 22, are of suciently large cross-sectional dimensions and high tensile strength safely to withstand tightening of the nuts to an extent where the tension in the bolts is of such large magnitude yas to compel the peripheral lengths of the jacket sections spanned by them, and with it the remaining periphenal lengths thereof, thermally to expand along and in 4substantially uniform engagement with the cylinder periphery, compelling the adjacent parting surfaces 30 of the jacket sections to creep toward each other and reduce the gap between them. In fact, the nuts 34 are tightened at such exceedingly high torque that the bolts 32 are at non-operational cylinder temperatures expanded within their elastic limit at least to an extent to remain at the highest operational cylinder temperatures `under sufficient tension to prevent any significant separation of the 1'acket sections from the cylinder periphery. 'The bolts 32 thus tact as powerful springs which, while not counteract-ing peripheral therrnal expansion as such of the complemental jacket sections 22, force them to respond to their thermal expansion and contraction by bending and unbending to whatever extent is necessary to retain them at all operational and also non-operational cylinder temperaturen in sufficiently yuniform form-fit and, hence, engagement with the cylinder 12 to preclude any significant temperature ydifferential penipherally of the latter. Also, the bolts. 32 `are over the axial extent of the complemental jaclcet sections 22 spaced sufficiently closely to each other to retain the latter throughout their axial lengths in substantially uniform form-fit with the cylinder periphery 28 at all operational and non-operational cylinder temperatures. Of course, the forni-fit on the periphery 28 of the cold cylinder 12 of the complemental jacke-t sections 22 by virtue of the described machining of their inner wall surfaces 26 to this end, contributes significantly to the attainment of a substantially uniform lform-fit lof these jacket sections on the cylinder` periphery at all cylinder temperatures, so that j the jacket sections yare lin optimum heat-exchange relation with the cylinder uniformly throughout Iits periphery for heat-up in iminimum time as well as during normal operation.

Of course, the dimensions of the bolts 32 and nuts 34 and their axial spacing along complement-al jacket sections may vary considerably in la given extrude-r installation, and it would be impossible to establish any limits in these trespects. Also, the bolts and nuts may be of any metal of high tensile strength, with high-grade steel being the preferred metal. Finally, the magnitude of the torque applied to the nuts for tightening them to the extent of actually expanding the bolts within their elastic limit to the described end has never been calculated, nor is it Inecessary to calculate such torque for a given extruder installation since it is so much easier and reliable to tighten the nuts on the bolts initially to the point where experience can pretty well (foretell accomplishment of the desired end, with a check thereof being readily made on a test run of the extruder installation. Also, while the rod-type braces are in the present example bolts with nuts which are tightened on the bolts for the necessary operational tension in the latter, these rod-type braces may take other forms, such 4as bolts headed at both ends and introduced in a highly heated state laterally into forklike lugs on the jacket sections for shrinkage therein to the point Where they will have expanded within their elastic limit to the desired extent after cooling off.

To demonstrate the remarkable and certainly unexpected high uniformity of the form-fit of the present clamped-on complemental jacket sections on the cylinder periphery at all cylinder temperatures, including the highest operational temperatures, there is given, by way of example only and by no means by way of limitation, the following data on test runs of two extruders which were alike in all respects, except that the modified extruder had the jacket sections mounted on the cylinder in accordance with the present invention while the unmodified extruder had the jacket sections mounted on the cylinder in conventional fashion. Each cylinder was of the same grade steel, and had a length of 120" and inside and outside diameters of 6 and 9.5", respectively. Both cylinders were disposed horizontally and the complemental jacket sections had their confronting parting surfaces along the tops and bottoms of the cylinders. Clamped on each cylinder were 8 heat-exchange jackets, with each jacket having two complemental cast aluminum sections of 13% length of which the inner and outer radii of their substantially semi-circular walls were 4% and 61/s, respectively, with both of these walls formfitting the cylinder periphery and being at their confronting parting surfaces spaced apart 1A". The mounted jackets were equally spaced apart axially of the cylinder, and all jacket sections had the same uniform external cooling fin arrangements on their walls. Each pair of complemental jacket sections had 6 clamp-on bolts of the same grade of steel and of 3A size, with 3 bolts extending across each of the two pairs of confronting parting surfaces of these jacket sections and being spaced apart 41/4" axially of the cylinder. The bolts on the unmodified and modified installations were 21/2 and 8" in length, respectively, and were spaced from the cylinder axis 7%" and 61A", respectively.

On heating up both cylinders, serious thermal deflection of the cylinder of the unmodified installation was noted at approximately 500 F., while no thermal defiection of the cylinder of the modified installation was noted at the same temperature. Thus, a dial indicator placed on top of the unmodified jacketed cylinder midway of its length showed an upward deflection thereat of about 1A" at approximately 500 F., which clearly indicated more heat along the top than along the bottom of the cylinder as caused by the jacket sections gaping away from the cylinder along the bottom thereof while remaining in gravitational contact with the cylinder along the top thereof. Thermal deflection of the cylinder to this large extent also indicated early roughing of the feed screw therein under operational endthrust thereon.

Next, only one unmodified jacket and one modified jacket were mounted on the respective cylinders midway of their lengths so as to observe the heating effects of these single jackets on the cylinders. The results were most revealing. Thus, the unmodified jacket required 136 minutes to heat its cylinder to 500 F. while the modified jacket required only 76 minutes to heat its cylinder to the same temperature. Gaps between each jacket and its cylinder were also probed with feeler gauges at 500 F. cylinder temperatures. Thus, considering the tops `and bottoms of the cylinders to be the 12 oclock and 6 oclock points, respectively, of a time dial, the clearance between the unmodified jacket and its cylinder was found to be a maximum of .015 at 11:30 and 12:30 oclock and a maximum of .034 at 5:30 and 6:30 oclock, while the clearance between the modified jacket and its cylinder was found to be a maximum of .008 at 11:30 and 12:30 oclock and a maximum of .009 at 5:30 and 6:30 oclock. The clearances just given were measured as the cylinders reached 500 F. on their heat up. As the heat of the cylinders was ironed out at 500 F., the clearance between the unmodified jacket and its cylinder decreased to .012 at the top and to .025 at the bottom, while the clearance between the modified jacket and its cylinder decreased to .006" at top and bottom. The temperature differential at the top and bottom of the cylinder with the unmodified jacket near the ends of the latter was found to be a maximum of 35 F. which decreased to 20 F. after the cylinder reached 500 F., while the temperature differential at the top and bottom of the cylinder with the modified jacket near the ends of the latter was found to be a maximum of 5 F. which decreased to 0 after the cylinder reached 500 F.

It follows from the preceding that, as to both, heating and cooling these cylinders, the rate of heat-transfer between the unmodified jacket and its cylinder and beween the modified jacket and its cylinder is in the ratio of 76 minutes to 136 minutes or 1 to 1.8, meaning that the heating and cooling capacity of the modified jacket is approximately greater than that of the unmodified jacket.

Reference is now had to FIGS. 5 and 6 which show different braces 32a for clamping the complemental sections 22a of heat-exchange jackets 20a on a cylinder 12a. The present braces 32a are of strap-type, having semicircular intermediate lengths 50 and straight endlengths 52, of which the intermediate length 50 of each brace embraces the seinicircular wall 23a of a jacket section 22a and extends with its endlengths 52 through lug formations 42a on the opposite jacket section to receive therebeyond nuts 54. The jackets 26a of the exemplary length shown in FIG. are clamped on the cylinder 12a by four of these strap-type braces 32a of which two embrace the part-circular wall of one jacket section and the other two embrace the part-circular wall of the other jacket section. The arrangement of the straight end lengths 52 of these strap-type braces 32a in relation to the jacket sections 22a is similar to the described arrangement of the straight bolt-type braces 32 in relation to the jacket sections 22 (FIG. 3), and the nuts 54 are similarly tightened to such an extent that the braces will be longitudinally expanded Within their elastic limit so as to retain the complemental jacket sections 22a in substantially uniform form-tit with the outer periphery of the cylinder 12a at all operational and also non-operational cylinder temperatures.

The invention may be carried out in other specic ways than those herein set forth without departing from the spirit and essential characteristics of the invention, and the present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.

What is claimed is:

1. In an extruder, the combination with a steel cylinder having an axis and an outer periphery concentric with said axis, of heat-exchange jackets on said cylinder substantially throughout its length, each jacket having two cast complemental sections of a metal of better heat conductivity and of a larger coeicient of expansion than the steel of the cylinder, with each section having a substantially semi-circular wall with an inner surface and heating means within an internal zone of said wall over the greater part of its semi-circular extent, and said complemental sections fitting with the inner surfaces of their walls said cylinder periphery and having narrowly spaced Cil braces each anchored with its opposite ends to the cornplemental sections, respectively, of a jacket and extending across said confronting parting surfaces of their Walls at a greater distance thereat from the cylinder axis than said inner zones of said sections, and further extending beyond each of said parting surfaces for a distance of over half the radius of said cylinder periphery, said metal braces solely securing said complemental jacket sections to the cylinder and having a smaller coeficient of expansion than the metal of said jacket sections, and said metal braces being longitudinally expanded within their elastic limit when the cylinder is non-heated, whereby said section walls are forced to bend in peripheral expansion into substantially uniform form-lit with said cylinder periphery at all operational cylinder temeratures.

2. The combination in an extruder as set forth in claim 1, in which the wall of each jacket section is formed with integral outer cooling ns substantially over its semicircular extent.

References Cited in the tile of this patent UNITED STATES PATENTS 205,271 Hyatt June 25, 1878 1,225,584 Cooney May 8, 1917 1,379,929 McLeroth May 31, 1921 1,868,990 Peelle July 26, 1932 2,435,426 Davies Feb. 3, 1948 2,705,342 Hendry Apr. 5, 1955 2,813,302 Beck Nov. 18, 1957 2,820,132 Krause Jan. 14, 1958 2,904,664 Rothacker Sept. 15, 1959 OTHER REFERENCES British Plastics Publication, March 1957, pages 113. 

1. IN AN EXTRUDER, THE COMBINATION WITH A STEEL CYLINDER HAVING AN AXIS AND AN OUTER PERIPHERY CONCENTRIC WITH SAID AXIS, OF HEAT-EXCHANGE JACKETS ON SAID CYLINDER SUBSTANTIALLY THROUGHOUT ITS LENGTH, EACH JACKET HAVING TWO CAST COMPLEMENTAL SECTIONS OF A METAL OF BETTER HEAT CONDUCTIVITY AND OF A LARGER COEFFICIENT OF EXPANSION THAN THE STEEL OF THE CYLINDER, WITH EACH SECTION HAVING A SUBSTANTIALLY SEMI-CIRCULAR WALL WITH AN INNER SURFACE AND HEATING MEANS WITHIN AN INTERNAL ZONE OF SAID WALL OVER THE GREATER PART OF ITS SEMI-CIRCULAR EXTENT, AND SAID COMPLEMENTAL SECTIONS FITTING WITH THE INNER SURFACES OF THEIR WALLS SAID CYLINDER PERIPHERY AND HAVING NARROWLY SPACED CONFRONTING PARTING SURFACES; AND STRAIGHT ROD-TYPE METAL BRACES EACH ANCHORED WITH ITS OPPOSITE ENDS TO THE COMPLEMENTAL SECTIONS, RESPECTIVELY, OF A JACKET AND EXTENDING ACROSS SAID CONFRONTING PARTING SURFACES OF THEIR WALLS AT A GREATER DISTANCE THEREAT FROM THE CYLINDER AXIS THAN SAID INNER ZONES OF SAID SECTIONS, AND FURTHER EXTENDING BEYOND EACH OF SAID PARTING SURFACES FOR A DISTANCE OF OVER HALF THE RADIUS OF SAID CYLINDER PERIPHERY, SAID METAL BRACES SOLELY SECURING SAID COMPLEMENTAL JACKET SECTIONS TO THE CYLINDER AND HAVING A SMALLER COEFFICIENT OF EXPANSION THAN THE METAL OF SAID JACKET SECTIONS, AND SAID METAL BRACES BEING LONGITUDINALLY EXPANDED WITHIN THEIR ELASTIC LIMIT WHEN THE CYLINDER IS NON-HEATED, WHEREBY SAID SECTION WALLS ARE FORCED TO BEND IN PERIPHERAL EXPANSION INTO SUBSTANTIALLY UNIFORM FORM-FIT WITH SAID CYLINDER PERIPHERY AT ALL OPERATIONAL CYLINDER TEMPERATURES. 